This invention relates generally to accelerometers and more particularly to accelerometers which employ light as their sensing mechanism.
The need for accelerometers is increasing as sensors increasingly need to recognize movement and make appropriate adjustments. Examples of apparatus which require or whose capabilities are enhanced by accelerometers include: robotics, automobile safety alarms, aircraft warning systems, satellite guidance systems, and guidance systems for munitions.
Acceleration is usually measured in "g's" where one-g is the acceleration created by gravity, or 9.80 meters per second per second.
As is clear from the examples where acceleration needs to be measured, the measured range is from a few nano-g's (i.e. satellite guidance) to several thousand g's (i.e. munitions guidance).
Several apparatus have been developed over the years in an effort to measure the acceleration of a variety of apparatus. In more recent years, the techniques have begun to utilize optics as a method of measuring acceleration. Optical accelerometers have an advantage in that they can be easily coupled with electronic circuits to provide dynamic control and monitoring systems.
To this end, a variety of instruments have been developed. In U.S. Pat. No. 5,684,298, entitled "Photonic-based Sensing Apparatus Using Displacement Tracking of an Optical Beam in a Semiconductor", issued Nov. 4, 1997, to O'Conner et. al, an optical beam is created which has a gap therein. Two electronic sensors monitor the gap which gives an indication of vibration (e.g. changes in the gap's characteristics).
In another approach to the problem, U.S. Pat. No. 5,633,960, entitled "Spatially Averaging Fiber Optic Accelerometer Sensors" issued May 27, 1997, to Lagakos, describes the use of a stress preventing layer which surrounds a center portion which has a higher Young's Modulus than the stress preventing layer. As the center portion deforms, an embedded fiber's characteristics alter, allowing the acceleration to be monitored and measured.
While these techniques do provide a level of measurement, each apparatus is highly restrictive as to the range of measurement and in it's applicability to either end of the spectrum which is to be measured. Neither apparatus, however configured, is able to measure in the nano-g range; and neither apparatus, however configured, is able to accurately measure and survive in the thousands of g range.
Further, many of these techniques are especially susceptible to electromagnetic interference which causes any readings to be suspect.
It is clear that there is a significant need to improve the sensitivity of accelerometers.